1. Field of the Invention
This invention relates to the field of hydrocarbon production by Fischer-Tropsch synthesis and more specifically to the field of producing a synthetic middle distillate by thermal cracking Fischer-Tropsch wax and isomerizing a Fischer-Tropsch heavy middle distillate.
2. Background of the Invention
Natural gas, found in deposits in the earth, is an abundant energy resource. For example, natural gas commonly serves as a fuel for heating, cooking, and power generation, among other things. The process of obtaining natural gas from an earth formation typically includes drilling a well into the formation. Wells that provide natural gas are often remote from locations with a demand for the consumption of the natural gas.
Thus, natural gas is conventionally transported large distances from the wellhead to commercial destinations in pipelines. This transportation presents technological challenges due in part to the large volume occupied by a gas. Because the volume of a gas is so much greater than the volume of a liquid containing the same number of gas molecules, the process of transporting natural gas typically includes chilling and/or pressurizing the natural gas in order to liquefy it. However, this contributes to the final cost of the natural gas and is not economical for formations containing small amounts of natural gas.
Formations that include small amounts of natural gas may include primarily oil, with the natural gas being a byproduct of oil production that is thus termed associated gas. In the past, associated gas has typically been flared, i.e., burned in the ambient air. However, current environmental concerns and regulations discourage or prohibit this practice.
Further, naturally occurring sources of crude oil used for liquid fuels such as gasoline and middle distillates (such as kerosene, diesel fuel, and home heating oil) have been decreasing, and supplies are not expected to meet demand in the coming years. Middle distillates typically include heating oil, jet fuel, diesel fuel, and kerosene. Fuels that are liquid under standard atmospheric conditions have the advantage that, in addition to their value, they can be transported more easily in a pipeline than natural gas, since they do not require energy, equipment, and expense required for liquefaction.
Thus, for all of the above-described reasons, there has been interest in developing technologies for converting natural gas to more readily transportable liquid fuels, i.e. to fuels that are liquid at standard temperatures and pressures. One method for converting natural gas to liquid fuels involves two sequential chemical transformations. In the first transformation, natural gas or methane, the major chemical component of natural gas, is reacted with oxygen to form syngas, which is a combination of carbon monoxide gas and hydrogen gas. In the second transformation, known as the Fischer-Tropsch process, carbon monoxide and hydrogen are converted into a mixture of organic molecules containing carbon and hydrogen. Those organic molecules containing only carbon and hydrogen are known as hydrocarbons. In addition, other organic molecules containing oxygen in addition to carbon and hydrogen, oxygenates, may be formed during the Fischer-Tropsch process. Hydrocarbons having carbons linked in a straight chain are aliphatic hydrocarbons and may include paraffins and/or olefins. Paraffins are particularly desirable as the basis of synthetic diesel fuel.
Typically, the Fischer-Tropsch product stream contains hydrocarbons having a range of numbers of carbon atoms and thus having a range of molecular weights. Therefore, the Fischer-Tropsch products produced by conversion of natural gas commonly contain a range of hydrocarbons including gases, liquids and waxes. Depending on the molecular weight product distribution, different Fischer-Tropsch product mixtures are ideally suited to different uses. For example, Fischer-Tropsch product mixtures containing liquids may be processed to yield gasoline, as well as middle distillates (such as kerosene, diesel fuel). Hydrocarbon waxes may be subjected to an additional processing step for conversion to liquid and/or gaseous hydrocarbons. Thus, in the production of a Fischer-Tropsch product stream for processing to a fuel, it is desirable to obtain primarily hydrocarbons that are liquids and waxes and that are nongaseous hydrocarbons (e.g., C5+ hydrocarbons).
High quality diesel is a desirable product from the Fischer-Tropsch process. The high quality diesel is typically prepared by hydrocracking Fischer-Tropsch wax and blending the hydrocracker product with the diesel range components produced directly in the Fischer-Tropsch process. The hydrocracking reaction is typically accompanied by paraffin hydroisomerization, which typically produces a diesel having improved cold flow properties. However, drawbacks include the diesel having a decreased cetane number. Further drawbacks include the cost accompanied by the catalysts used in the hydrocracking reaction.
Consequently, there is a need for a diesel product in the Fischer-Tropsch process having improved cold flow properties and cetane number. Further needs include reducing the costs of diesel production in the Fischer-Tropsch process.